CA1070455A - Molybdate orange pigment with improved thermal stability - Google Patents

Abstract

Abstract Molybdate orange pigment treated with from 0.1 to 30% of a boron compound. The boron-treated molybdate orange pigments when coated with dense, amorphous silica are particularly useful for coloring high temperature molding plastics.

Prior Art U. S. Patent 3,567,477 U. S. Patent 3,370,971 U. S. Patent 3,639,133

Description

~0704S~

The term l'molybdate orange" as used herein refers to the pigments ident~fied by the Colour Index Number CI 77605 in the "Colour Index", second edition, 1956, pub-lished jointly by the Society of Dyers and Colourists, England, and the American Association of Textile ~hemists and Colorists~ United States. mey range in color from a relatively light masstone, strong and yellow tirt (small particle size) to a dark, red masstone and weak, red tint (relatively large particle æize).
Molybdate orange pigments have been prepared by a variety of methods, most o~ which involve precipitation of molybdate orange pigment from aqueous solutions of its constituent ions, i.e., lead, molybdate, sulfate and chromate.
Conventionally a solution containing soluble salts of molybdate, sulfate and chromate is mixed with a lead salt, in the form of an aqueous solutlon or a slurry depending on the solubility of the lead salt. After precip~-tation, but prior to isolation, the molybdate orange p1gmRnt is commonly tre~ted with silica or alumina or both to provide a loose porous coating on the surface of the pigment to enhance pigmentary properties, such aæ lightfastness, in paint compositions. me red shade molybdate orange pigments are conventionally further treated with an antimony compound, commonly antimony trichloride, after the treatment with silica and alumina but prior to isolation of the pigment.
A widely used method which produces molybdate orange pigm~nt of especially high strength is described in U.S.
Patent 3 567 477. According to the procedure of U.S.
Patent 3 56f 477 molybdate orange is produced by ~etting at high linear velocity a first aqueous salt solution containing _~_ ~' 1~70~

soluble salts of molybdate, sulfate and chromate into a second aqueous solutlon containing a soluble lead salt, thereby preclpltating the pigment, which is treated with silica and alumina in the conventional manner prior to isolation.
The ma~or contribution toward color of molybdate orange pigment resides in the formation of a solid solution contalning lead chromate. Likewise, many Or their short-comings are also attributable to the properties inherent ln the lead chromate component. Among these are (1) sensi-tlvity to alkalies and acids, (2) stalning in the presence of sul~ides and (3) darkening on exposure to light or to elevated temperatures.
E~rly attempts to overcome these deficiencies have usually involved special treatment of the Digments ~signed e1ther to nrovide a means of neutralizin~ the attac~ing agent and rendering it temporarlly ineffectlve or to pro-~ide a barrier agai,nst the read~ access thereof to the sensitive pi~ment ~article. The claimed im~rovements, 20 althou~h very ~requently demonstrable, have usually been onl,v Or academic interest in that thelr effectiveness has been short, lived and not of suffic~ent, duration to render no~sible the use Or these pi~ments in many applications, ,' particularly those appl~cations where a high degree o~
thermal stab~lity is required.
More recent and relatively successful attempts to , overcome these de~iciencies are described, e.g., in U.S. Pat-ents 3,370~971 and 3,639,133. These patents describe coated lead chromate pigments, which are chemlcally resistant, lightfast and relatively thermally stable. The pigments .

~(~7()~55 thereln described are prepared by applying a coating of dense, amorphous silica and optionally alumina to lead chromate pigment whlch has been precipitated and a~ter-treated with silica and alumina in a conventional manner.
The dense silica-coated lead chromate pigments are described generally as reslstant to darkening when heated to 300C.-320C. ln such plastics as polyethylene or polystyrene. A
dense sillca-co2ted molybdate orange pigment is described in Example 1 of U.S. Patent 3,370,391 as exhibiting no ~ignlficant discoloration up to about 250C. and minor dis-coloratlon up to 315C. in a thermoplastic resin, compared to a conventional molybdate orange which darkens noticeably at 230C.
Althou~h the dense silica-coated lead chromate pigmsnts do show signlflcant improvement in thermal stabillty when compared w~th uncoated pir,ment and gel-coated pigment known in the art, the reslstance to darkeni~g exhlbited by these pigments at 300C.-320C. is low enough under many - processing condltions to necessitate the use Or temperatures ~rom 30C.-60C~ lower than the stated range to insure ade-quate color inte~rlty. The resistance to darkening is rurther decreased ln proportion to the length Or time the pigment ls exposed to elevated temperatuPes. For example, a pigment which retalns color integrity when initially reachlng an elevated temperature can darken considerably after being malntained at that temperature ~or several minutes.
The need to avoid elevated temperatures and extended exposure to elevated temperatures has a particularly adverse impact on the thermoplastics industry. In the thermoplastics lndustry pigmented thermoplastics are o~ten held at the extrusion temperature for as long as 30 minute~ prior to extrusionJ during which time the pigment can significantly darken. In addition, the speed Or further processing depends primarily on high temperature of the thermoplastic. In the processing of thermoplastics the uæe of temperatures even 30~C. below 300C., not to mention 60C. below, can decrease polymer flow and increase residence time o~ the polymer in the mold to the extent that process erficiency su~fers and overall productivity decreases.
This invention provides for boron-treated molyb-date orange pigment whlch, when coated with dense, amorphous silica, retains a signiricantly higher degree Or color integrity at high temperatures than conventional dense-~ilica-coated molybdate orange pigments, while retaining and, in some cases surpassing, the degree of ch~mical resistance and lightfastne~s exhibited by the conventional molybdate orange pigmenta According to the invention there i8 provided an improvement in a proces~ for preparing molybdate orange pigment in aqueous medlum by contacting an aqueous solution containlng soluble salt~ Or molybdate, sulfate and chromate wlth a ~alt of lead to form an aqueous slurry of said pig-ment and applying porous silica and alumlna to the precipi-tated pigment. The improvement re~ide~ in adding from 0,1 to 30~ by weight o~ a soluble boron compound, calculated as boric oxide, based on the we~ght o~ the final base pigment, to the aqueou~ medium. me phrase "final base pigment" as used herein refers to the ~oron-treated molybdate orange pigment with porous silica and alumina applied thereto. me ~, ~ ' ?
. ,, ~

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boron compound utllized in this process is a water-soluble lnorganic oxygen-containing boron compound which ls capable Or formlng boric acid upon contact with aqueous media. For example, boric acid, boric oxlde and acldified water-soluble borates, such as alkall metal borates in acidic solu-tlon, are useful in the process of the inventlon.
The boron compound can be added prior to precipi-tatlon of the molybdate orange pigment to partially replace the chromate or sulfate in the initlal precipitate mixture or can be added at any time after precipitation of the pig-ment, l.e., prlor to, during or after the application of porous silica and alumina to the precipitated pi~ment. The total amount of porous silica and alumina used in the practice Or the lnvention is not critical. However, in common prac-tlce from 0.5 to 3.0% by ~ei~ht Or porous silica (calculated as SiO2) and alumina (calculated as A1203), based on the welght o~ the final base pigment, is recommended to insure a coating which imparts the best pi~mentary properties to the final base pigment.
To treat molybdate orange pigment prior to precipl-tation, it is preferred that the boron compound be added to tAe aqueous solution containing difisolved molybdate, sulfate and chromate to avoid any premature reactlon with lead. When the boron compound is added prior to precipitation it is neces~ary to add only from 0.1 to 1% by weight of the boron compound, calculated as ~23~ to insure optimum properties in tAe final product. Amounts of boron compound greater than about 1% by weight are not recommended for use prior to pre-cipitation as such lar~er amounts may alter the color Or the pl~ment. Addltion Or the boron compound after precipitation ~070~55 requlres rrom 1 to 30% by weight of the boron compound, calculated as B203, to insure desirable properties. Larger amounts of boron can be added after precipitatlon without adversely affecting product quality, but such excess amounts do not improve the nroduct.
The boron-treated molybdate orange pigment of the invention contains porous silica and alumina, pre~erabl~ in a total amount from 0.5% to 3.0% by weight, calculated as SiO2 and A1203, respectively, and based on the weight of the flnal base pigment, and from 0.015% to 0.10% of boron, calculated as B203 and based on the weight of the ~inal base plgment.
The boron-treated molybdate orange pigment Or the invention is particularly useful as base pigment to which a coating of dense, amorphous silica and, optionally, alumlna is applied accordlng to procedures well-known in the art, e.g., U.S. Patent 3,370,971. Although the pigments Or the invention sho~ at least comparable physical and chemical properties relative to untreated molybdate orange pigments, a signiricant improvement in chemical resistance and especially in thermal stabillty is exhibited when the pigments o~ the invention are coated with dense, amorphous silica. The coating o~ dense, amorphous silica is preferably applled in an amount from 2% to 40% by weight, calculated as SiO2 and based on the total weight of the dense silica-coated pigment. Compared to conventional molybdate orange pigments coated with dense, amorphous silica, the boron-treated molybdate orange pigments of the invention coated with dense, amorphous sillca show a marked improvement in color integrity at temperatures as high as 320C.

llr70455 The aqueous ~edlum ~rom ~hieh lybdate orange pigment ~8 preelpitated should contaln salts of molybdate, sulfat~ and ehromate whlch are at loast derately ~oluble and pre~erably hlghly 801ubl~ in aqueous solut~on. For roasons o~ high ~olubllity, eeonomy and availabllity alkali ~etal ~alt~, partleularly sodiu~ ~alts, o~ mol~bdate, ehro-~ate and sul~ate are prererred. The lead ~alt 1~ eom~only addod to the ~queous ~edium in the for~ of an aqueo w 801u-tlon or an aqueou~ slurry, d¢pending on the sol~bllity o~
th~ l~ad ~alt used. Although ~oluble lead ~alt~ aro pre-forred to lnJure eo~plet~ reaetlo~ ln~oluble load ~alte, ~uch ~ lead carbonate, can be w ed to form lybdate orange plgment. ADong the 801ublo load ~alt~ w eful in ~orming mol~bdate oran8o pigment are lead nitrate and lead acetate.
Because Or the extremcly lo~ ~olubllity of lyb-date orange plgment ln aqueo w medla, tho plgm~nt precipl-t~tes ~ery rapldlr fro~ a solutlon of $t~ constltuent ions.
To racllitate tho ~ormation o~ a complete Jolld ~olutlon of load chrom~te, load molybdate and lead ~ulf~e, ~hich constltute~ pure ~o~ybdate or~nge pigment, lt i~ deslrable to mis ~n aqueou~ ~olutlon Or mol~bdat~, chromate and ~ul-~ate ~ith a separate aqueouJ solutlon or 81urr~ of lead salt. It 1~ ~re~erred that the m~sing be done under condl-tlon~ ~hlch promot- rapid intlm~te contact Or the t~o 801u-tion~, such as the ~xing mRthod de~cribed ln U.5. Patent 3,567,477, to m~nimiz~ the amount of ~nocl~nic lead chro-m~te ~hlch preclpltate~ ~ith the desired solld ~olut~on.
The boron compound utilized in the proce~s of th~ ~nv~ntio~ t co~Yenientl~ added to the aqueous medium in which th~ pigment 1~ prepared in the for~ o~

107~45~;

an ~queou~ ~olut~on o~ the boron compound. ~ varlety o~
801uble boron compounds are w erul ln thls proces~, e.g., borlc a¢ld, borlc o%ldo and acldi~led al~al~ metal borates.
A maJor crlterlon for the boron compound, in addltlon to ~ater-~olubility, 1~ that lt be capable of ~orming boric aeld ln the ~queo w solutlon in which the pigment i~
preclpitated.
~ hethor the boron compou~d 1~ added turlng or follo~lng prcclpltatlon of the plg~e~t, it i~ ~ssential that the plg~nt rocelve the convontlonal a~ter-treatments ~ith porou~ ~llica and alumina ~ order to achieve impro~e~
properties in th~ flnal pigment. No signl~lcant lmprovement in the propertles of melybdate orango plg~ent i8 observed ~h~n thc boron compound i~ e~ployed a}one~ i.e., ~ithout appllcation o~ coatlng~ Or poro w slliQa and alnm~na.
It is not clesr, at thl~ ti~e, e~actly ho~ the boron compound e~r~cts tha impro~ement~ ln porous slllca and alumina coated ~olybdate orange plgment. Electron micrograph~ Or the Pigoant~ of the in~entlon indlcate a 20 oub8tantlall~ contlnuou~ coating on t~e molrbdate orange pigoent partlcles, co~par~d to a noncontlnuou~ coating on pig-~ent partlcles prepared without a boron compound. Whether tha coatlng obser7ed i~ compo~ed or ~e~eral layer~ or an intl-m~ta comblnation ~ not kno~n~ ~t 1~ pos~bl~ that a mO19CUlar monolayer Or lead borate i~ form~d on the molyb-datc orange plgment particle~ ~hlch impro~e~ the adh~slon o~ the ~oro w ~llica and alumina coatlng~ applled a~ter precipltation o~ th~ pigEont. Although the role of the boron co~pound9 which i~ at }sa~t partially in the ~orm Or borlc acid i~ solution, in the co~Fos~tion~ of the ~nventlon i8 not 10'7l)~$

rully understood, it apparently by some mechanism facilitates the appllcatlon of porous silica and alumina ts the molybdate orange plgment, which when coated with dense, amorphous silica exhibits much improved thermal stability.
In any event, the after-treatment with porous silica and alumina which rollows precipitation of the molybdate orange pigment can be applied ln the conventional manner well-known to those skilled ln the art. The porous silica coating is commonly applied prior to isolation of the precipitated pigment by adding an aqueous solution Or a soluble sillcate compound capable Or forming hydrous silica in water, e.g., sodium silicate. The alumina is applied by adding a soluble aluminum compound capable of forming hydrous alumina in water to the aqueous slurry of pigment. Among the aluminum compounds used are aluminum halides and hydrated aluminum sul~ate, which may be added dry or in a~ueous solution.
As stated hereinabove, the signiricant improvement in the thermal stability of the molybdate orange pigmen~
Or the inventlon is achieved ~lhen a coating of dense amorphous silica, and optionally alumina, is applied to the final base pigment. All of the known procedures for applying a dense silica coating to pigment involve using plgment whlch has been isolated ~rom the aqueous slurry in which lt was precipitated. The isolation procedure conven-tlonally involves filtration, washing and, optionally, drylng. For the boron-treated pigment prepared according to the invention, isolation prior to dense silica coating is preferred to achieve pigment having the highe~t thermal stability. The silica coating can be applied to the final ~7~ 4 S ~

base plgment ln an amount rrom 2~ to 40% b~ ~elght, b~sed on the totsl weight Or the dense sllica-co~ted pl6~ent, by a v~riety o~ conventional methods, such as described in U.S.
Patent~ 3,370.9 n and 3,639,133.
In one m~thod a dllute sodium B~ llcate solutlon (about 3% by Neight of S102~ i8 pa~sed through a bed o~
cation ~xchange re~in ln the h~drogen form o~ ~ufricient capaclty 80 that 811 Or the ~odlum ion 18 r~oved to glv~
a sillclc acid effluent ~ith a pH Or 2.9-3.3. Such a 801u-tion i8 only m~d~rately stabl~, but will ko~p ~or se~ralhours ~ithout gelation if maintaln~d at a lo~ temper~ture (c108c to O~C.). A ~uitabl~ amount of ~uch a 8ilicic acid solution i8 then added ~lowly (3-5 hours) to the alkaline plgment ~lurr~ at 90-95C. whlle maintalning the pH in the rango of 9.0-9.5 by th~ perlodic additlon of incr~ments of a dilut~ alkali such as NaOH ~olutlon. Ob~iou~l~, the ezact method Or malntalnlng the pH ln the de~lre~ r~nge i8 not lmport~nt ~nd lt could be done by malnta~ning the pre~ence of a suitabl~ bu~fer. Such a mothod ~ould be 3ub~ect, ho~c~er, to the limltations lmpo~ed by the ~reoence of ~odium ion or other m~talllc ions, and the 8impl~ per~odic addit~on o~ a sultablQ alkall ha8 many points in its favor.
In another method equi~alent amoUntB or a dllute ~odlum ~ilicate solutlon (5.r% S102) and a dllute 8ulfurlc acld ~olution (3.18% ~ SO~) are added s$multaneo w ~y to the hot (+ 95C.) alkaline plgm~nt h~Urry over a ~ubstan-tlal period of time ~3-~ hours) wh~le maintaining the pH
in the range Or 9.0-10.0 by p~r$odic addition o~ 8~all amn~nt~ of dilute alkall (~aOH so1utlon for instance). This i8 th~ pra~erred m~thod o~ operation, und the vfiriation~

~0'70~5S

mentioned earller may easily be applied thereto. Thus, the pH may be allowed to go below 9.0, perhaps as low as 6.o or 7.0, without completely destroying the desired effect;
nevertheless, the results are better in the higher range.
The reactlon time can be reduced to as little as one hour without serious detriment. Although the temperature of reaction may be as low as about ~0C., this causes some tendency toward the formation of free SiO2 in gel form;
hence it is highly de~irable to keep the temperature above at least 75~. The exact conc~ntratio~s Or the solutions used are not important except that they should be relatively dilute and the amounts used should be essentially chemi-cally equivalent.
A third method for applying a dense,amorphous silica coating to pigment involves adding all Or the sodium sillcate solution to the pigment slurry rrevlously made alkal~ne, as with N~40H. This gives a pH of about 11.0, near the maximum permisslble and, after heating the mixture to about 75C., a 5% sulfuric acid solution is added slowly (1 to 1.5 hours) to give a final pH Or about 7.8 to 8Ø
~n addltional heating period up to about one hour is desir-able in this procedure. The concentrations used and time of addition are not critical.
In each of these methods, the dense silica-coated product can be highly d~spersed and difficult to filter wlth a considerable tendency for yleld loss during the filtration and a tendency to result in hard products on drying.
It is preferable, therefore, that a flocculation step be introduced. Such a step, although not necessary to the preparation of pigments of the desired characterlstics, 107~45~
offer~ many advantage~ ~uch a8 impro~ea ee8~ Or riltration and ~ashlng, elimination Or yleld 108~e~ during ~lltratlon and lmproved te~ture of th~ r~sultlng pigment. A variet~ o~
methods kno~n in the art, ~uch a8 those dcscribed ln U.S.
Patent 3,370,971 are w erul in lsolating the dense s$1ica-coa~ed boron-treated plgm~nt of thls lnventlon. A prererred m~thod ~or facilitating isolatlon Or the product $8 to add a polyvalent mntal salt, especially an aluminum salt such as ~odlum aluminate, $n aqueous ~olution, to th~ slurry of dense ~illca-coated pigment prior to isolatlon.
Th~se methods o~ forming the ~illca layer on the surface o~ the lead chromate pigment particles ha~e some reature in co n, viz. that the ~illca i8 added a~ "actlYe slllcan. Nhen sodlum slllcat~ i8 acldified, 8ilicic acid iB formed, probably inltlally as orthosll~cic acid, Sl~OH)4.
Ho~ever, thi~ product tends to polym~rize the reaction of t~o s~lanol group~

(- si -to ~orm a silo~ane group (- Sl - O - Sl -) Under acidlc conditions, this polymerizstlon proceed~
rapldly untll a predom~nant proportlon of the sll~nol group6 pre~ent ha~e been u~ed ~p ~n the ~ormation Or silo~ane bonds. Ihe poly~er thu~ formæd ha~ a htgh molec-ul~r w~ight and ~ defined a8 "~nactlven Under the con-dit~on~ of m~derate al~sllnity used ~n the exa~ples below, there i8 a lo~ dogree o~ po~ymerl~atlon ~herein the conden-s~tion between sllanol group8 ha8 proceeded to only a limlted extent leaving th~ silica in an "actlve" form whlch readily deposlts on the surface of the plgment particles present. It i8 not intended to imply that thcre i8 no polymerlzatlon nor that the process o~ poiymerization ~ 8 compl~tely inh~bited but, under the condltions speci~led, the sllica iB ~n a state of lo~ polymerization and, thus "actlven, for a surftclent tlme to brin~ about depo~ition in the dense, amorphou~ rorm on the ~urface o~ the pigment partlcles .
Th~ quantity Or slllca to be applied in the plg-ment coating can ~ary over a con~iderable range depend~ng on the intended end u~e of the pigment. For use in e~truded hot thsrmoplastlc resin~ from 15 to 32% by ~eight o~ silica 18 desirable, and amounts up to about 40g by ~eight can be used without adv~rsely a~feetlng plgment color. In applica-tions where e~tremely high thermal stability i8 not re~uired, amounts of ~ ca as lo~ ~8 2~ by ~elght pro~ide improved chomlcal reslstance and llghtfa~tnes~, m us, for purposes of this invention the dense, amorphou~ 8ilica coatlng can be in the range of about 2% to 40% by we~ght of the flnal den~e ~lllca-coated pigment. When alumlna ~ 8 al~o present the quantity of A1203 can be ~ar~ed from 0.25~ to 2% by weight of the final plgment.
In one ~m~od~ment of th~ ~n~ention ~oron-tr~ated molybdate orange pigm~nt ia prepared by contactlng a flr~t aqueQu~ ~olution ~ontaln~ng a mixture o~ dl~olved sodium 8~1t~ of chromat~, mo~ybdate and sulfste at a pH rrOm 7 to 9.5 ~ith a second aqueou~ ~olutlon contain~ng dissolved le~a n~trate at a pH ~rom about 3 to 4 to precipitate the plgment. The resulting pi~ment slurr~ i8 stirred for ~7~

several minute6 to facilitate cr~stal growth, commDnly known as development~ a~ter ~hleh the ~lurry i8 n~utraliz~d.
After neutralization an aqueou~ solution of horlc oxide i8 added to the slurry. The ~mount o~ boric o~ide added iB
from 1 to 30~ by weight on a dry basis, based on the weight o~ the final ba~e plgment, m en separate aqueou~ solution~
o~ ~od~um sll~cate and h~drat~d aluminum ~ulf~te, resp~ct~ely, are added sequentially to the r~action slurr~. Th~ pH
of the slurry i~ ad~u~ted to from about 5.5 to 6.5. The boron-trested l~bdate orange pigment i8 isol~ted from the slurry by flltratlon, washed wlth ~ater and dried.
In another ~bodiment Or th~ inv~ntion, boron-treated lybdate orange pigment i8 prepared ky contactlng an aqueou~ lead nltr~te ~olutlon a8 d~crlbed aboYe ~ith an aqueous solution con~aining in adaition to the sodlum 6alts ll~ted above ~rom O.l to 1% by weight of borlc oxide on a dry basis, ba~d on the weight Or the ~inal base plgment. Th~ resulting pigm~nt slurry i8 treated ~ith ~lllca and alu~ina and the flna~ product iB lsol~ted a#
descrlbed abov~0 The ~ollo~ing example~ ar~ lnt~nded to lllu~trste the ln~entton. me term nparts" aB used herein ref~rs to parts by ~eight. The physical and chemlcal properties o~
all exemplifi~d pigment~ are determlned according to the procedures de~cribed in Ex~mple l.
Example 1 Sol~tion A i~ prepar~d ~y dis~olving 386~2 parts of Pb(~03)2 ln 3160 par~s o~ wat~r and th~ pH i~ adJu~ted to 3~0-3,1 at ~ te~peratur~ of 67F.-69F. (l9~C.-21C.). Solu-t~on B is pre~red by dis~olvln~ l46.l part~ of Na2Cr207.~ ~ 0, 22,4 p~rts of Na2MoO4 and 7.2 parts o~ Na2S04 ln 3103 part~
of water, and the p~ i~ adJusted to 7.4-7.6 at a temp~rature Or 67F.-69~. (19C.-21C.).
To precipltate the pigment, Solution B i8 lntroduced under the surface Or Solutlon A over a period of 17-19 mlnutes with agltation. Chlorlde ion concentratlon 1B adJu~ted to develop tint and strength in the eon~entional manner by adding 57.0 parts of NsCl, after Rhich the s~urry 18 ~tlrred ~or 15 minutes. Th~n an aqu~ous solutlon contalning 16.0 parts of B203 (4-% ~Y ~e~ght, ba~ed on elght of ~lnal bas~ pigment) i~ added to the slurry and ~tirring 18 continued Por three minutes. Porous 8~11Ca 18 then applled to the ~igment by ada~ng 18.3 part~ Or ~Oaium ~illcate (29.5% of S102, SiO2/-Na20 - 3.25, such as DU Pont Technical Grade ~o. 9). Then an aqueous solution containing 28.~ part~ o.~ hydr~ted alum~-num ~ulfate (equivalent to 56.1% o, A12(S04)3 or 17.1% of A1203 ) 18 add~d to the slurry. The 81urry i8 then neutra-li~ed by addlng 19.0 parts Or ~odium earbonate. ~fter a ~ew mlnutes of ~dditional stlrring, 5,2 parts o~ antimony o~ldc dissol~ed in h~drochloric acld i8 added to the 81urry, after ~hlch the pH of the slurry i~ ad~u~ted to 5.8-6~0.
me re~ulting brilliant r~d shade mol~bdate orange plgment i8 isolat~d from the slurry ln the conventional manner by filtration, wsshlng and drying.
Electron m~crographs o~ th~ boron-tre~ted molybdate orange pigm~nt ~how B ~u~stanti~lly continuous outer coating.
Electron ~crogr~phs o~ a ~on~entional ~olybdate orang~ pre-~ared in the ~entlcal manner, e~cept ~or the omi~ion of the bor~c oxlde, ~how a d~continuo w coatlng.
~hen the boron-treated molybdate orange plgment ~no4~

and the conventlonal molybdate orange pigm~nt are separately dlspersed in a conventlonal alkyd coating C0~pO8~ tlon vehlcle and panel~ coated with the resulting comFosltion are exposed to light, a similar de OE e~ of lightrastness i8 ob~er~ed.
When 81milar panels ~re e~po~d to a 10% aqueous solution Or sodlum hydrozide and a 1% aqueous solution o~ sodium ~ul~ide, respectl~ely, the boron-treated molybdate orange pigment and the conventional molybdate orange pigment exhlb~t comparable resistance to spotting and color lo~
To test ~or re~l~tance to darkonlng in thermo-pla~tic at high temperature~ the boron-treated molybdate orange pigment and th~ conventional molybdate orange pig-ment are sepsrately mi~ed ~ith solid granular polystyrene and sub~ected to can rolllng for ten minutes, accordlng to the method de~cribed in U.S. Patent 3,639,133, then t~o-roll milled (0.016~ clearance) into sheets. The ~heets are cut and placed in an extruder at temperatures ranging fro~
204C. to about 320C. Prlor to extruslon the hot poly-~t~rene i8 mQlntained at temperature ror 20 minutes. When the polystyrene i~ e~truded, the poly~tyrene containing the boron-treated lybdate orange 8ho~ resistance to darkening ~rom 204C. to about 320C. similar to the poly-styrene containing the conventional molybdate orange pigment.
E~ample 2 A paste i~ prepered by mlxing 150 part~ o~ the boron-treated pigment prepared accordlng to the procedure Or Exsmple 1, 20 part~ o~ the ~odlum sillcate de~cribed ~n Example 1 ~nd 447 parts of ~ater. The mixture ~8 ~urther diluted with 790 parts of ~ater ~or a total of about 1237 ~)7(~45~

parts of ~ater. me pH o~ the resulting slurry ~8 then ad~usted to 11.5 ~lth 8 5% aqueous solution o~ sodlum h~dro~ide and the result~ng pigment slurr~ i~ heated to 9oo C.
In separate containers the ~ollo~ing solution~
are prepared:
(1~ 115 parts of the above-descrlbed sodium sllicate i8 ~dded to 530 part~ of ~ater,

(2) 17,3 part~ Or 96.o% sulfuric acid 1B
added to 800 parts Or ~ater.
These solutions are th~n added ~imultaneously to the plgment ~lurr~ prepared above o~er a perlod of throe hour~ for ~olution (1~ and 3-3/4 hours for solutlon (2), ~hile malntainlng the temperature at 90-95C. through-out the addition. Arter completion Or the addltion of solutions (1) and (2), an aqueous Jolution contalning 10.0 ~arts of ~luminum ~ulfate in 100 parts Or ~ater i8 aaded to the 81urry. The slurry 18 stirred rOr Pi~e minutes and the pH i8 ad~u~ted to 4.0-4.2 ~ith 5% aqueous 801ution oP
sodlum hydroxlde. The product i8 then ~llter~d, ~a~hed ~ith ~ater to a reslstance of 5~000 ohms and dried overnight at Z20F. (100.4C.) to glve a ~illca-coated red shade molyb-date orange pigment Or ~uperior propertie~.
~ hen tested for lightra~tness and chemlcsl re~i~tance ln the m~nner described in Example 1, the den~e sillca-coated me~l~date orsnge treated ~th boric o~de ho~ comp~ra~le ligh~astne~ but lm~ro~ed chemical reslstance relativ~ to a con~entional dense sllic~-coated mo~ybdat~ or~nge pigmen~ prepared ~n the identical manner, except for the omi8~10n 0~ the boric oxid~.

4s~

~ hen mlxed ~lth po~y~tyrene and extruded at high temperatures in the manner des~rlbed ln Esample 1, the polystyrene containing the dense slllca-coated molybdate orange plgment treated wlth boron ~how~ ~ub~tantlally no darkening at temperatures up to about 280C. and onlr very ~llght darkening at 320C. By comparl~on the pol~-styrene containing the conventional den~e 8~ lica-coated lybdate orange pigment begins to darken at 280-C. and exhiblts significant darkening at 320C.
Example 3 The procodure of E~ample 1 i8 follo~ed, except that 32.0 parts of boric oxide (8% by wei~ht, based on ~ei p t of the final base plgment) i~ used.
LightfastneE~, chemical re~istance and thermal ~tQbillty Or the re~ulting plgment ar~ 8imilar to those Or the boron-treated pigment descrlbed in E~a~ple 1.
E~ample 4 The procedure Or E~ample 2 is follo~ed, except that 150 parts o~ the molybdate orange pigmen~ of Ex~mple 3 iæ u~ed.
Llght~astness, chemic~l re~istance and thermal stabillty of the re~ulting d~nse ~ ca-coated pigment are slmilar to thos~ o~ the boron-treated dense 8~ lica-coat~d pigment describ~d ln Exampl~ 2.
kxam~le 5 The procedure of Example 1 i8 ~ollow~d, except that 4.0 part6 Or boric oxide ~1% by weight, b~sed on ~eight of ~inal ba~e pigment) ~ wed.
Lightfa~t~sæ~, chemical resi~tance and thermal stabillty of the resulting pigment ar~ simll~r to t~o~e 10'7V45~:i o~ the boron-treated pi~ment de~crlbed in Exa~ple 1.
Example 6 The procedure of E~ampl~ 2 i~ followed, c~cept 150 parts of the molybdate orange pigment Or E~aQple 5 18 u~ed.
Llght~astnoss, chemical resletance and thermal ~tablllt~ Or the re~ultlng den~e sllica-coated pigment are similar to those of the boron-tr~ated d~nse sillca-coated plgment de~cribed in E~ample 2.
E~amplo 7 Solution A 18 prepared by di3solving 349.6 parts o~ Pb(N03)2 in 3174 part~ of ~ater and the p~ i8 adJusted to 4.0-4.1 at a temperature of 67F.-69F. (19C.-21~C.).
Solutlon B i~ prepared by dis~olving 132.25 parts Or Na2Cr207.2 ~ 0, 3.7 parts Or Na2S04 and 22.3 part~ of Na2MoO4 in 3174 p~rts o~ ~ater, and the pH i8 ~d~w ted to 9.0-9.4 at a temperature of 67F.-69F. (19~.-21C.~.
To precipltate the pigment, Solutlon B i8 lntro-duced under the ~urface o~ Solutlon A o~er a period of ~e~eral ~inutes. Then 2.51 parts o~ h~drochloric acid and 28.0 p~rts o~ ~odium chloride i~ added to the resulting 81urry. ~ter stlrring the 81urry for 15 min~tes, 10.3~
parts o~ sodlum carbon~te 1~ added to neutralize the ~lurry.
Then ~n a~ueoug solutlon containlng 34.0 part~ o~ boric ox~de (10~ b~ we~ght, ba~ed on the ~elght of the ~n~l base pigm~nt) i8 added to the slurr~ and stirred ~or three minute~.
Porou~ gllica i8 then applied to the pig~ent by adding 20.7 parts o~ the sodium ~ilicate deserlbed in Example 1. Then 2~.3 p~rt~ of the hydrated alum~num sulrate de~cribed in E~ample 1 ~ added to the ~lurry, and the pH is ad~u~ted to 50 8-6. o.

-20~

7U4~S' A brilllant yellow shade molybdate oran~e i~ ~o-lated from the slurry by filtratlon, wa~h1ng snd dr~ing.
Chemical resistance and thermal stability of thls pigment are slmilar to those of the red shade lybdate orange plgment treated ~ith boric oxlde described in Example 1~
The color and lightrastness of this plgment ar~ comparable to a con~entional molybdate orange pigment, prepared a~
describ~d above except that the addition of boric oxide $Q
omitted.
E%ample 8 Th~ procedure Or E~ample 2 i~ follo~d, except that 150 ~arts of the yello~ sh~de m~lybdate or~nge pig-ment Or Example 7 i~ used.
The chemical resistance and thermal ~tabillty of the resultlng dense s~lica-coated pigment are similsr to those Or the boron-treat~d dense sillca-coated plgmRnt de-scrlbed in Example 2.
Example 9 The procedure of E~ample 7 i8 followed, e~cept that 17.5 part~ of borlc oxide (5% by weight, based on weight o~ flnal base plgment) i8 used.
The chemical re~istance and thermal stability of the resulting yellow ~hade molybdate orange pigment are ~imllar to tho~e o~ the boron-treated pigment described in E~ample 1.
Exa~ple 10 Th~ procedure o~ Example 2 i~ ~ollo~e~, except that 150 part~ of the yellow shade molybdate oran~e plg ment of E~ample 9 i~ used.
The c~emical re~tance and thermal stability 1071~4S~

of the resulting dense silica-coated pigm~nt are s~mllar to those of the boron-treated dense silica-coated pigment descr~bed ln E~ample 2.
E~ample 11 The procedure of Example 7 i8 followed, e~cept that 68.o part~ Or borle o~lde (20~ b~ weight~ based on ~e~ght of rinal base plgment~ ls used.
The chemical resi~tance and thermal ~tabillty of the resulting yellow ~hade lybdate orange plgment are similar to those of the boron-trested pigment descr~bed in Example 1, E~ample 12 The procedure o~ Example 2 ~s followed, except that 150 parts Or the yellow shade molrbdate orange p~g-ment o~ Example 11 i8 used.
The ch~mical resistance and th~rmal stability of the result~ng den~e silica-coated pigment are slmilar to those o~ the boron-treated dense ~ilica-coated plgment described in Example 2.
E~ample 13 Solutlon A 18 prepared as de~cribed in Example 1.
Solution B is prepared by dissolvlng 142.8 part~ of 07.2H20, 7.16 p~rts of Na2S04, 22.4 parts of Na ~ 04 and 1-56 part~ of 8203 (0.4% by weight, based on ~eight of final ba~e pi~ment) in 3103 part~ of water and the pH ~
sd~usted to 7.4 at a temperature of 67~.-6~F. ~1~C.-215.).
To precipitate th~ pigment, Solution B i~ intro-duced under the surface of 501ut~on A o~er a perlod of 17-19 minutes ~lth agitation. ~hen 57.0 parts of NaCl i~ added ~ollo~ed by ~tirring ~or 15 m$nute~ The re~ul~ing slurry ~7~ 4 S ~

i8 then neutralized by adding 11.4 part~ o~ sodi~m carbonate.
Arter a fe~ minutes o~ additlonal stirring, porous silica i8 Applled to the pigment by adding 18.4 parts of the sodlum s~llcate descrlbed in E~ample 1.
After ~tirring ror three minute~ 28.5 parts of the hydrated sluminum sul~ate described in Example 1 is ~dded to the 81urry. Then an 8qUeoU8 solution contsining 19.0 part~ of ~odium carbonate i8 added to the slurr~.
A~ter a re~ minutes of additional stirring the plgm~nt iB
treated with antimnny trichlor~de as described ln ExRmple 1.
The resultlng brilllant red shade molybdate orange pigm~nt i8 iBolAted in the conventional ~anner by filtratlon, ~ashing and dr~ing. Upon analysis and evalua-tion as described ln Example 1, this pigment i8 found to have chemlcal resistance and thermal stabil~ty 8imilar to the molybdate orange pig~ent treated Nith borlc oxide after preclpitatton descrlbed in Example 1, Example 14 The procedure of Ex~mple 2 1~ follo~ed, except that 150 parts of the molybdate or~nge pigment Or E~ample 13 i~ u~ed.
The chemic~l res~stance and thermal stabillty Or the resultlng dense s~lica-coated plgment are ~imllar to tho3e of the denge ~lica-coated pigmRnt treated ~ith boric oxlde after precipit~tion described in ExAmple 2.
E~mple 15 The proc0dure of Ex~mp7e 13 ~ 8 ~oll.o~ed, except that 4.1 partB 0~ Na2S04 and 146.16 part~ Or Na2Cr207.2 ~0 are u~ed in Solution B.
The chemical resi~tance and thermal stability o~

1~7045~

the r~sulting pigment are slmilar to tho~e Or the boron-treated plgm~nt deecribed in E~ample 1.
Ex~mæle 16 The procedure Or Example 2 18 followed, except that 150 part~ o~ the mol~bdate orang2 pigment ~r Exa~ple 15 i8 u~ed, The chemical resl~tance ~nd thermal st~billty of the re~ultlng den~ silica-coated pigment are ~imllar to tho~e o~ the dense ~illca-coated pigment treated ~ith boric oxide after precipitation descrlbed in Example 2.
Example 17 The procedure o~ Example 13 iB rollo~ed, except that 146-16 part~ o~ Na2Cr207-2H20, 16.9 partP of Na ~ 04 and 7.16 part~ of ~a2S04 are used ln Solutlon B.
me chemical resistance and thermal ~tab~lity of the resultln~ plgment are aimilar to those described for the boron-treatcd plgment Or Example 1.
E~ample 18 Th¢ ~rocedure Or Example 2 i8 followed, e~cept that 150 part~ of the molybdate orange pigment o~ Example 17 18 ~sed.
The chemlcal re~istance and thermal ~tability of the resulting den~e silica-coated pigment are s~milar to those o~ the den~e sllica-coated pigment treated wlth boric oxlde after preclpitation described in Example 2.

Claims (8)

The embodiment of the invention in which an ex-clusive property or privilege is claimed are defined as follows:

1. A process for preparing molybdate orange pigment in aqueous medium by contacting an aqueous solution containing soluble salts of molybdate, sulphate and chromate with a salt of lead to form an aqueous slurry of precipitated pigment and applying porous silica and alumina to said pre-cipitated pigment, wherein the improvement comprises adding from 0.1 to 30% by weight of a soluble boron compound, calculated as B2O3, to said aqueous medium.

2. Process for preparing molybdate orange pigment according to Claim 1 wherein said boron compound is a water-soluble inorganic oxygen-containing boron compound which forms boric acid upon contact with aqueous media.

3. Process for preparing molybdate orange pigment according to Claim 1 wherein a coating of dense, amorphous silica is applied to said molybdate orange pigment.

4. Molybdate orange pigment containing porous silica and alumina and from 0.015% to 0.1% by weight of boron, calculated as B2O3.

5. Molybdate orange pigment according to Claim 4, coated with dense, amorphous silica.

6. Molybdate orange pigment according to Claim 5 wherein the dense, amorphous silica is in an amount from 2%
to 40% by weight, calculated as SiO2 and based on the total weight of the silica-coated pigment.

7. Molybdate orange pigment according to Claim 4 wherein the total amount of porous silica, calculated as SiO2 and alumina, calculated as A12O3 is from 0.5 to 3% by weight.

8. Molybdate orange pigment according to Claim 7 having a coating of dense, amorphous silica in an amount from 2 to 40% by weight